It is known to form and transfer images by non-impact means. One non-impact technology involves the use of ribbons comprising substrates having coated thereon a solid meltable ink.
Conventional thermal printing over the years has proven itself as one of the major printing technologies, featuring low hardware cost and high reliability. Its major drawback has been the need for special, chemically treated paper. Conventional thermal paper has been greatly improved over the past few years. Yet, such papers remain expensive, less convenient to handle, and less permanent and archival than "plain" papers. In conventional prior art thermal printing, heat is applied to special paper with a chemical coating designed to turn dark at a specified threshold temperature. Heat is used to cause a chemical reaction in the coating of the imaged paper.
The distinguishing feature of thermal transfer printing is that heat is used to cause the transfer of ink onto a receptor surface. Under digital control, the ink transfer is selective so that characters or other programmed patterns are printed.
As described in the prior art literature or performed by the products introduced to date, ink is coated onto an intermediary material such as film, paper or ribbon. A thermal head, in contact with the ink carrier which is in turn in contact with the surface to be printed, generates sufficient heat to cause the ink on the back of the "ink donor material" to transfer to and penetrate the receptor surface. One variation described in the literature, but not yet implemented as a product, is preheating the ink and selectively transferring it from a cylinder.
The ink and substrate are the keys to thermal transfer.
Various known prior art substrates have been used including Mylar, condenser paper, other polyesters and conductive polyurethanes. Of these, two substrates are most commonly used, Mylar and special condenser paper. Mylar is a registered trademark of DuPont. The Mylar ranges in thickness from 3.5 to 6 microns and the special condenser paper ranges in thickness from about 10 to 13 microns. Condenser paper is used in thermal line printers in widths ranging from about 33 to 267 mm. (The low tear strength of the condenser paper precludes its use for narrower ribbons.) Polyester film like Mylar is found in thermal serial printers in common widths of 6.35, 8.0 and 12.7 mm. The length of both condenser paper and polyester film depends upon the space available in the printer or ribbon cassettes.
There are other substrates known to be adapted for use in thermal transfer ribbons. Some of these are described in U.S. Pat. Nos. 4,103,066; 4,269,892; 4,291,994 and 4,320,170. In U.S. Pat. No. 4,103,066 a polycarbonate resin containing conductive carbon black is used as a substrate for a resistive ribbon for thermal transfer printing. In U.S. Pat. No. 4,269,892 a polyester substrate is disclosed containing from about 15% to about 40% by weight of electrically conductive carbon black.
Many polyester resins are known to the art and are commercially available. As examples of useful materials there may be mentioned the Vitel polyesters. Vitel is a trademark of Goodyear Tire and Rubber Company and is of a class of polyesters which are linear saturated resins containing few free hydroxyl units. Examples of such materials are PE207, PE222 and VPE4583A. Mylar 49000 is another polyester which is known to have given good results when used in prior art systems. A commonly used material is Estane 5707-FI, a polyester which has been crosslinked with isocyanate. Estane is a trademark of the B.F. Goodrich Company. The substrate layer of the ribbons of this prior art patent are from about 8 microns to about 35 microns in thickness.
In U.S. Pat. No. 4,291,994 a substrate is disclosed which comprises a mixture of from about 50% to 90% by weight of a polycarbonate with from about 10% to about 50% by weight of a block copolymer of bisphenol A carbonate and dimethyl siloxane with from about 15% to about 40% by weight of electrically conductive carbon black.
In U.S. Pat No. 4,320,170 an electrically conductive substrate is discussed which comprises polyurethane having predominantly only urethane functional groups and an electrically significant amount of conductive carbon black.
In one type thermal transfer process heretofore used the transfer ribbon in addition to the above-described substrates includes a hot melt ink layer. When a current is applied to the ribbon the resistive material from the substrate heats up causing the ink at that point to transfer to the printing surface or receiver paper. Best results in these prior art systems have been obtained when a smooth finish paper is used, such as Xerox 1024 and 4024 papers.
Further, other thermal transfer processes are described in U.S. Pat. Nos. 2,713,822 and 3,744,611. In these prior art processes the hot melt ink layer melts at from about 85.degree. C. to about 90.degree. C. These ink transfer materials have relatively poor smudge resistance and are known to also have a poor rub resistance. In addition, many of the thermal transfer inks do not melt sharply and have other below-discussed deficiencies. Commercially available inks, as discussed in the literature on thermal transfer, are based on a mixture of low melting waxes. By definition, waxes gradually soften as they are heated although many have a crystalline component which melts sharply giving a sudden increase in fluidity. Use of waxes leads to a ribbon or film which requires relatively low energy for transfer but has the deficiencies associated with the softening below the crystalline melting point. These deficiencies may include blocking (i.e., adhesion of the ink to the back side of an adjacent layer), offsetting of the ink to material in contact with it at ambient temperatures and poor resolution or edge acuity in the transferred image. In addition, it is desirable to have available a solid meltable ink that has low energy requirements for transfer and permits non-blocking of the roll of ribbon up to at least 45.degree. C. In addition, it would be desirable to have no offsetting of the ink onto material it comes in contact with such as the substrate side of the ribbon when it is rolled at temperatures up to 45.degree. C. Also, a ribbon providing good image quality when transferred, especially image density, resolution, edge acuity and smudging is needed for present special requirement systems. Most of these desirable characteristics are primarily a function of the hot melt ink used.